Various methods are known which impart fire retardance to a resin. These methods are classified broadly into an addition type method and a reaction type method. In the former method, antimony hydroxide, antimony trioxide, boron compounds, bromine compounds, and phosphorus compounds are used, and flame retardation can be attained through a simple method. However, the former method is disadvantageous in that the compound bleeds from the surface of the resin during prolonged environmental exposure or when exposed to a heated atmosphere thereby spoiling the surface appearance and lowering the flexibility. In the latter method, an epoxy group, a vinyl group, an ester forming functional group, or the like is introduced as a reactive group into a monomer, an oligomer, and a polymer of a bromine compound, a phosphorus compound, or the like, and these fire-retardant groups are incorporated into the molecular skeleton or incorporated into a polymer molecule through a reaction between polymers. The reaction type method not only is free from bleeding but also can impart flexibility and therefore is a useful method which can eliminate the drawbacks of the former method. However, in the latter method, complicated procedures are required in order to incorporate a fire-retardant monomer into a polymer skeleton because no increase in the degree of polymerization can be attained by an ordinary method. The addition of a reactive fire-retardant oligomer or polymer is a simple expedient to overcome such a difficulty. However, this method unfavorably brings about an interaction between the additive and the base polymer during kneading, which leads to lowering in both the molecular weight and the mechanical properties. This tendency is particularly significant in polyesters and polyamides.